PHOTONICS INSIGHTS
understanding of physics’ most fundamental concepts. Einstein showed that Newton’ s theory of universal gravitation, in which gravity is an instantaneous force between masses, is actually an approximation of a more general theory. The new theory of general relativity became the framework through which we study the universe at large scale, stars, and new, exotic objects discovered in the 20th century, such as pulsars, black holes, and neutron stars. Soon after publishing his famous equations relating energy and matter to spacetime curvature( the field equations of general relativity), Einstein discovered that a specific solution to these equations involved waves. In other words, the curvature of spacetime, produced by violent phenomena in the universe, can propagate through space, just as electromagnetic waves do. When an extremely energetic phenomenon shakes spacetime, it vibrates, such ripples of the spacetime, called gravitational waves spread throughout the universe. However, spacetime is extremely rigid and the distortion produced by gravitational waves is incredibly small. We now know that the most powerful sources observed by LIGO and Virgo produced a change in the detector baseline( which is a few km) smaller than the size of an atomic nucleus. Such extremely precise measurement has been done for the first time by LIGO in 2015 [ 1 ], and then jointly by LIGO and Virgo in 2017 and it was possible after more than 50 years of experimental development of interferometric detectors. These instruments, known as Michelson interferometers, operate on a simple principle: a laser beam is split into two by a beam splitter and the two paths travel along two perpendicular arms, each several kilometers long. At the
Figure 1. Simplified schematic of a gravitational-wave detector showing the entry points of vacuum fluctuations that give rise to shot noise.
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